Broadcast television channels in the United States are assigned channel numbers between 2 and 83, inclusive. Until recently the cable television channel numbering system differed from the broadcast television channel numbering system, in that cable television channels were numbered from 2 to 99. Despite this difference, all broadcast television channels and cable television channels could be selected by entering, at most, two digits via a keyboard, mounted on the television receiver itself, or on a remote control unit. This limit of two digits per channel number has allowed the development of television receivers in which the tuner is controlled to select the desired channel immediately upon receiving the second digit of the channel number. The RCA CTC-140 color television receiver chassis, manufactured by Thomson Consumer Electronics, Inc., Indianapolis, Ind., is an example of such a system.
Recently in the United States, frequency space has been allocated for cable television channels which are numbered in excess of 99, thereby requiring a three-digit keyboard data entry system. The Electronics Industries Association (EIA) has recommended the following numbering system for cable channels.
______________________________________ CABLE PICTURE CHANNEL # CARRIER FREQUENCY BAND ______________________________________ 65-89 313.25 MHz-613.25 MHz UHF 90-94 619.25 MHz-643.25 MHz UHF 95-99 (historically called A-1 to A-5) Low VHF 100-139 649.25 MHz-885.25 MHz UHF ______________________________________
Cable channel numbers 95 to 97 reside within the commercial FM radio broadcast band (88 MHz-108 MHz) in the United States, and are usually unused because of interference problems which may occur between the desired television signals and the undesired FM radio signals.
Two prior three-digit channel selection system are known. In the first of these known systems, the keyboard includes an ENTER key for signaling the completion of a channel number entry. Such a system is known from the MBC-300 remote control unit manufactured by Zenith Corporation. There are four disadvantages to this approach. First, viewers have become accustomed to the above-mentioned immediate reaction of the television receiver in tuning a channel upon receipt of the last digit entered by the viewer. This desirable feature is lost in a system which employs an ENTER key. Second, operation of the ENTER key undesirably adds a separate keystroke to the selection of all channels, requiring three keystrokes to select most channels, and four keystrokes to select channels greater than 99. Third, the cost and complexity of the keyboard, keycode encoding and keycode decoding is increased by the addition of the ENTER key. Fourth, it requires an additional key on keyboards which in recent years have become more and more crowded as new features have been added. For example, the keyboard of the RCA CRK-55 remote control unit includes 80 keys. Adding an ENTER key to such complex keyboards is undesirable, in that a user may have to search among all those keys for the additional ENTER key.
In the second of the known prior three-digit channel selection systems, a remote control unit includes a "100s" key. The operation of the 100s key enables a three digit channel number entry mode, by causing the entry of the digit 1 in the leftmost place of the three-digit channel number. Such a remote control unit is manufactured by Magnavox Corporation and bears model number UR111MX. This approach also exhibits the above-mentioned feature of an immediate reaction of the television receiver by tuning a channel upon receipt of the last digit entered by the viewer. However, it has the three other disadvantages discussed above with respect to the separate ENTER key.